Articles having surfaces capable of spreading water, and thus preventing the formation of water droplets on the surface of the article, are desirable for a variety of uses. For example, transparent plastics used in misty or humid environments, such as windows of greenhouses, should avoid the formation of light-reflecting water droplets which reduce light transmission. A water-spreading surface on these materials helps to maintain their transparency and minimizes undesirable streaking.
Water-spreading characteristics are also desirable on traffic signs that employ retroreflective sheeting. Retroreflective sheeting has the ability to return substantial quantities of incident light back towards the light source. Such light transmission into and back from a retroreflective sheeting is impaired by clinging raindrops and/or dew.
A prominent form of precipitation that affects light transmission is dew formation. Dew can be particularly problematic because it occurs predominantly at nighttime when the retroreflective sheeting is operative. When present on a traffic sign in the form of small beaded water droplets, dew can disrupt the path of incident and retroreflective light. This can make information on the sign very much more difficult for passing motorists to read. In contrast, when the dew is spread out smoothly as a transparent layer over the surface of the retroreflective traffic sign, the information on the sign is easier to read because the resulting thin smooth layer of water does not significantly misdirect the path of incident and retroreflective light to as great an extent.
A problem with water-spreading surface coatings, especially silica-based coatings, is the highly complex nature of the surface chemistry, reaction chemistry and solution chemistry of colloidal silica and colloidal silica films. For example, the interaction of ions with the silica surface is not fully understood despite extensive study (See Iler, “The Chemistry of Silica,” John Wiley, 1979 p. 665.) Despite such difficulties, a silica-based water-spreading film that has enhanced durability is provided in accordance with the invention described below.